The present application claims priority from German Patent Application No. 100 57 048.8, filed Nov. 17, 2000, and hereby incorporated by reference herein.
The present invention is directed to a method and a device for generating a color profile using the determination of the mapping Axe2x80x2 of a device-independent process space Q(m) of dimension m, m being a natural number, to a device-dependent process space Kxe2x80x2(n) of dimension n, n being a natural number.
Tables constitute an essential part of color profiles. They enable device-independent color values, such as {Lab} or {XYZ} values, to be converted into device-dependent color values, such as CMYK, and vice versa, for various color reproduction strategies. The tables contain sampled values of multi-dimensional mappings, such as {CMYK} as a function of {Lab} or vice versa, in each instance for a regularly spaced grid of points in the particular input space. Functional values of points, which do not lie on the interpolation grid, are typically determined by employing diverse, suitable interpolation methods among the adjacent interpolation points. Generally, the color profiles are calculated with the aid of proof-printed test formes containing many different color fields, typically between 200 and 1000 individual colors. These test colors are gauged colorimetrically. They represent a sampling of the printing process. The mappings described in the tables of the profiles are, for the most part, very complicated, and generally are not able to be globally described in a closed form by mathematical formulas. Typically, the device-independent process space Q(m) is not linear, but metric. However, it may be represented accurately enough locally as a linear space. The device-dependent space K(n) is metric and is linear for important applications.
Often, printing processes for which profiles are created, are not constant, but change their properties over time, whether it be continuously or discontinuously. Thus, a previously generated profile may still be valid for the process, but only with limited accuracy, or it may no longer be valid at all. Often the influencing quantities, which lead to such changes, are not precisely known, neither with respect to their number nor their values. Possible influencing quantities include, inter alia: changes in paper, printing inks, and screen resolution; use of printing additives (thinners, printing jelly or the like), other surface processing (lacquer coat, powder, or the like), altered ambient conditions, such as temperature, air humidity, altered printing sequence or chromatic and achromatic composition.
The profiling of a printing press using a printing test entails considerable effort, time expenditure, as well as certain costs. Such a profiling must be specifically tailored to a parameter set of various conditions. Altering one parameter in the parameter set usually means that a completely new test is necessary. This is particularly true when a plurality of colors or papers is used in a printing company for one production lot. In other words, to avoid the deviations that a profiled edition is subject to when changing over from a previous to a specific, new parameter set, it is necessary to profile the printing press for the specific parameter set of the subsequent printing conditions.
A method and a device for generating a color profile is known, for example, from the German Patent Application No. 43 35 143 A1. This document describes a method and a device for converting color values of a first color space, typically {Lab}, into color values of a printing color space characteristic of equipment and systems in a printing process for the color image processing, typically {CMYK}. In order to adapt the color separation values to the particular printing process, for the color values {Lab}, the color separation values {CMYK}, assigned according to function, are analytically calculated, by approximation, as a printing table, taking into consideration the printing colors, the colormetric properties of the printing material and of the printing process parameters, and subsequently calculated using a color value calibration {Lab}- greater than {Lxe2x80x2axe2x80x2bxe2x80x2}, in that correction values are determined by comparing characteristic color values in a test table to colormetrically measured color values of one of the test tables prepared as a print proof or proof.
Further related art is constituted by German Patent DE 43 05 693 C2. This publication describes a color calibration method where color values of a first color space which are dependent upon an input unit are converted into the color values of a second color space, and where, inter alia, to obtain color values of the first color space, a test sample containing a number of defined test colors is measured out. The color separation values of the first color space are converted into the color values of the second color space, which are assigned according to function, on the basis of a table. To acquire differential color values, from which appropriate color correction values are calculated, the color values of the second color space obtained by calculation are compared to the color values of the second color space obtained by measuring out the corresponding test colors.
German Patent Application No. 198 44 495A1 describes a method for profiling and calibrating a digitally controllable printing press having a permanent printing form, where a color profile is automatically selected from a local or non-local pool of color profiles, e.g. one that is accessible over the Internet, including in some instances, one-dimensional correction. To prepare definitive data, a data-processing device automatically employs a profile that corresponds exactly to the active state of the machine; in other words, a color profile corresponding to the correct color space calculation or calibration. To this end, a machine state predicted for the instant of printing and, with knowledge of the operating materials, a machine profile that most closely proximates the printing job, are determined at the instant of data preparation.
An object of the present invention is to provide a method for generating a color profile that entails less outlay.
The present invention provides a method for generating a color profile using the determination of a mapping Axe2x80x2 of a device-independent process space Q(m) of dimension m, m being a natural number, to a device-dependent process space Kxe2x80x2(n) of dimension n, n being a natural number, for a first set of specific process parameters. The mapping Axe2x80x2 is represented by a concatenation of known mapping A from Q(m) to a device-dependent process space K(n) of dimension n, n being a natural number, for another set of specific process parameters, which differs, at least in one element, from a first set of specific process parameters; and either of a mapping TK from K(n) to Kxe2x80x2(n), or of a self-mapping TQ by Q(m).
To produce a new color profile each time the properties, and thus the parameters, of a printing process change, it is not necessary to measure out all color fields again, when one can assume that, in response to typical variations, the entire characteristic of the printing process does not change, but rather only certain parameters exhibit, for the most part, slight changes. Instead of producing a new color profile in the same manner as the previous one, new measured values are determined for only a few test colors. The characteristic parameters of the variation are determined, using a given calculating rule, from the comparison to the old values, which may be derived from the old color profile, for example, directly or via a simple calculation. Using a variation model determined on the basis of these characteristic parameters, a new color profile, which better conforms to active process state, may then be calculated from the old color profile, thereby improving the printing quality. The present invention provides a method and a device which make it possible for a color profile to be generated rapidly and accurately. To this end, an approximative, analytical conversion of an old color profile having specific first process parameters to a new color profile is performed arithmetically for specific other process parameters, such as color, paper, resolution, or the like. In this manner, the benefit of a reduced outlay for profiling is achieved. Thus, by employing the method and/or the device of the present invention, the need for a complete printing test for the profiling may be eliminated, when it comes to changing the color scale, the printing material, the screen count, or corresponding process parameters, since only a small number of new measured values is required in a test. The method and/or the device of the present invention achieve simplification, shorten the time required, and reduce technical outlay, making the printing press or the pre-press unit easier to use and reducing the costs entailed in their application.
In accordance with the method of the present invention, from a known old color profile, a new color profile may be defined on the basis of the measurement of the color spectra and, respectively, of the {Lab} values of the printing material, as well as on the basis of the process colors for solid areas or flat tints; in an advantageous embodiment, also screen tints may be defined for the primary and/or secondary and/or tertiary colors. For this, the existing assignment rule is converted between values in the device-independent process space Q and the device-dependent process space K, the so-called look-up tables, with knowledge of the spectra of the new colors and/or the new printing material, and the spectra of the corresponding old colors and/or the printing material. The new colors may be a matter, for example, of a new four-color set, an altered screen count, or also, however, of a modified four-color set composition, or the sequence of printing the individual colors.
The present invention also provides a device for generating a color profile, using a mapping Axe2x80x2 from an m-dimensional, device-independent process space Q(m) to an n-dimensional, device-dependent process space Kxe2x80x2(n), n, m being from the natural numbers, wherein the device has an arithmetic unit or processor (55), which, from a known color profile, thus from a mapping A from a device-independent process space Q(m) to a device-dependent process space K, generates a new color profile, thus the mapping Axe2x80x2, using a calculating rule, which describes a concatenation of mappings, which includes a mapping from K(n) to Kxe2x80x2(n) and/or from Q(m) to Qxe2x80x2(m).